Is bill colouration in wild male Blackbirds (Turdus merula) related to biochemistry parameters and parasitism?

Abstract

Studies of signalling and sexual selection in birds are increasingly focussing on the regulation of the expression of carotenoid-based ornaments. Brighter coloured ornaments are preferred during mate choice and are thought to signal resistance to parasites, immune capacity and health status. As the colouration of integuments is a dynamic trait, changes in colouration potentially reflect changes in individual condition. The bill of the male Common Blackbird (or Blackbird, Turdus merula) has become a model for studying the implications of carotenoid-coloured integuments in birds, and Blackbird bill colouration has been found to be related to reproductive ability and immune capacity. However, the relationship between bill colouration and parasites and health in this species remains unclear. We have analysed the association between bill colouration, parasites (blood and intestinal parasites) and health status indicators (standard haematological and plasma biochemistry variables) in free-ranging male Blackbirds during the breeding season. Bill colouration was found to be related to body condition, health status, stress and hydration and nutritional status, but the presence or load of the parasite groups studied was not found to be related to bill colouration. Moreover, parasites showed no clear aggregation patterns. Our results suggest that certain physiological constraints-rather than parasite infection-are the main cause of variability in the colouration of male Blackbird bills.

Male Red-backed Fairywrens appear to enhance a plumage-based signal via adventitious molt

“…Variation in food quantity or quality, dietary carotenoids, dietary cholesterol (the carrier of carotenoids in the blood stream), and hydration can all cause changes in carotenoid bare-part coloration (McGraw and Parker 2006, Velando et al 2006, Pérez-Rodríguez and Viñuela 2008, Morales et al 2009b, López et al 2011. Body size and body condition are commonly correlated with carotenoid bare-part color (Bright et al 2004, Rosen and Tarvin 2006, Biard et al 2010, Doutrelant et al 2013, and in our dataset, 70% of species tested showed evidence for a positive correlation…”

The role of bare parts in avian signaling

“…Variation in food quantity or quality, dietary carotenoids, dietary cholesterol (the carrier of carotenoids in the blood stream), and hydration can all cause changes in carotenoid bare-part coloration (McGraw and Parker 2006, Velando et al 2006, Pérez-Rodríguez and Viñuela 2008, Morales et al 2009b, López et al 2011. Body size and body condition are commonly correlated with carotenoid bare-part color (Bright et al 2004, Rosen and Tarvin 2006, Biard et al 2010, Doutrelant et al 2013, and in our dataset, 70% of species tested showed evidence for a positive correlation…”

“…Additionally, it appears that social interactions could possibly have such a strong influence on bare-part color that it obscures relationships with condition. In the study above, the bill color of female American Goldfinches reflected the outcome of competitive interactions but not induced energetic stress, despite the existence of a correlation between carotenoid-based bare-part color and the same stress indicators in other species (Pérez-Rodríguez and Viñuela 2008, López et al 2011, Svobodová et al 2013.…”

“…Environmental toxins can disrupt carotenoid bare-part coloration within 1 mo (Bortolotti et al 2003, Pérez et al 2010a, 2010b, and challenges to both the innate and acquired immune system typically result in duller carotenoid bare parts within days or weeks (Faivre et al 2003a, Alonso-Alvarez et al 2004, Peters et al 2004a, Gautier et al 2008, Rosenthal et al 2012; but see Velando et al 2014). Carotenoid bare parts may respond rapidly (within 6.5 hr) to stressors, including capture and captivity (Rosen and Tarvin 2006, Biard et al 2009, Sternalski et al 2010, Rosenthal et al 2012, Vergara et al 2015, and their color reflects indices of recent stress such as corticosterone levels or the heterophil:lymphocyte ratio (Roberts et al 2007, Pérez-Rodríguez and Viñuela 2008, López et al 2011, Kelly et al 2012. Carotenoid coloration also predicts the strength of individual immune responses and the ability to clear parasite infections (Faivre et al 2003b, Peters et al 2004b, Dawson and Bortolotti 2006, Velando et al 2006, Stirnemann et al 2009.…”

“…For instance, dietary limitation and physiological trade-offs are typically invoked to explain the honesty of carotenoidbased signals, yet carotenoids appear to be less limiting for bare parts than for plumage. Diet is a better predictor of plumage color than bare-part color across species (Olson and Owens 2005); and gut parasites, which inhibit the absorption of carotenoids and the production of lipoproteins (Allen 1987), appear to have a relatively more severe impact on plumage (Hatchwell et al 2001, Mougeot et al 2005, Biard et al 2010, Martínez-Padilla et al 2010, López et al 2011). Because many bare parts occupy a small surface area in relation to plumage ornaments, they may require fewer carotenoids to color.…”